The present invention relates generally to coatings for corrosion protection, and more particularly to an article having such a coating.
Gas turbine engines are well developed mechanisms for converting chemical potential energy, in the form of fuel, to thermal energy and then to mechanical energy for use in propelling aircraft, generating electric power, pumping fluids etc. One of the primary approaches used to improve the efficiency of gas turbine engines is the use of higher operating temperatures. In the hottest portion of modern gas turbine engines (i.e., the primary gas flow path within the engine turbine section), turbine airfoil components, cast from nickel or cobalt based alloys, are exposed to gas temperatures above their melting points. These components survive only because cooling air is passed through a cavity within the component. The cooling air circulates through this cavity reducing component temperature and exits the component through holes in the component, where it then mixes with the hot gasses contained within the primary flow path. However, providing cooling air reduces engine efficiency.
Accordingly, there has been extensive development of coatings for gas turbine hardware. Historically, these coatings have been applied to improve oxidation or corrosion resistance of surfaces exposed to the turbine gas path. More recently, thermal barrier coating have been applied to internally cooled components exposed to the highest gas path temperatures so that the amount of cooling air required can be substantially reduced. Since coatings add weight to a part and debits fatigue life, application of the coating is intentionally limited to those portions of the component for which the coating is necessary to achieve the required durability. In the case of rotating parts such as turbine blades, the added weight of a coating adds significantly to blade pull, which in turn requires stronger and/or heavier disks, which in turn require stronger and/or heavier shafts, and so on. Thus there is added motivation to restrict use of coatings strictly to those portions of the blade, e.g., typically the primary gas path surfaces, where coatings are absolutely required.
With increasing gas path temperatures, turbine components or portions of components that are not directly exposed to the primary turbine gas path may also exposed to relatively high temperatures during service, and therefore may also require protective coatings. For example, portions of a turbine blade that are not exposed to the gas path (such as the underside of the platform, the blade neck, and attachment serration) can be exposed to temperatures in excess of 1200 F. during service. These blade locations are defined at 18 and 19 in FIG. 1. It is expected that the temperatures these portions of the blade are exposed to will continue to increase as turbine operating temperatures increase.
It is an object of the invention to provide a corrosion-resistant coating to prevent corrosion of components in regions not directly exposed to the hot gas stream.
It is another object of the invention to provide a corrosion-resistant coating to prevent stress corrosion cracking on portions of turbine blades which are not directly exposed to a hot gas stream.
It is yet another object of the invention to provide such a coating to protect against stress corrosion cracking of turbine blades in regions under the blade platform.